Use has been made of 3,5,5-trimethylcyclohexa-3-en-1-one (.beta.-isophorone) as a pharmaceutical or agricultural intermediate or as a material for a polymer such as polyurethane. The .beta.-isophorone is prepared by isomerization of .alpha.-isophorone, generally in the presence of an isomerization catalyst composed of an acid.
By way of example, Japanese Patent Publication No. 8650/1979 (JP-B-54-8650) discloses a method which comprises isomerizing .alpha.-isophorone with using an acid as an isomerization catalyst and distilling the isomerized product to give .beta.-isophorone, the acid being stable under the reaction conditions and having a pK value of 2 to 5 as well as having a higher boiling point than the isomerized product. As the isomerization catalyst, this literature uses an acid which is not thermally deformed at the isomerization temperature and which does not cause a side reaction, naming adipic acid as an example. The literature teaches that the continuous isomerization is generally conducted under atmospheric pressure, although it is also possible under reduced pressure, and that the reaction temperature should be set high in order to yield the isomerized product in as large an amount as possible per unit time. In this literature, .beta.-isophorone is obtained in a 94% yield according to the process which comprises the steps of boiling the mixture of isophorone and the adipic acid isomerization catalyst under atmospheric pressure for isomerization, distilling the thus isomerized .beta.-isophorone, and further rectifying the distillate under reduced pressure.
However, when the reaction is conducted under atmospheric pressure, the reaction temperature reaches 213 to 215.degree. C., which is a boiling point of .alpha.-isophorone. At such a high temperature, the isomerization catalyst is thermally decomposed and gives a by-product. For instance, the adipic acid isomerization catalyst is thermally decomposed and by-produces cyclopentanone, water and carbon dioxide.
As a result, the amount of the isomerization catalyst (e.g. adipic acid) decreases gradually, and the isomerized product is produced in a reduced amount per unit time. For these reasons, such a process requires an additional isomerization catalyst (e.g. adipic acid) to be added into the reactor in the course of the reaction. Further, the distilled .beta.-isophorone is contaminated, during distillation, with the by-products (cyclopentanone, etc.) resulting from the thermal decomposition.